Russia announced a unilateral, daily three-hour ceasefire, starting Thursday, to allow aid into the sprawling city in Syria. But CBS News correspondent Debora Patta reports it failed before it even began, as neither the rebel coalition nor the Syrian government appeared to endorse it.

The heavy fighting never stopped.

Overnight there were claims of another chlorine gas attack on rebel-held Aleppo. One witness described a chemical explosion and then the smell of gas.

"The children," he said, "all of us were choking."

Even if all parties agree to a ceasefire, the United Nations wants more time, insisting it needs 48 hours to bring desperately-needed resources to the war-weary people of Aleppo.

Syrian civil defense workers, right, help an injured woman after warplanes attacked a street, in Aleppo, Syria, in an image released June 8, 2016, by the Civil Defense Directorate in Liberated Province of Aleppo.

AP

The U.K.-based Syrian Observatory for Human Rights said Thursday that government aircraft had dropped barrel bombs in Aleppo's rebel-held Zabadieh neighborhood. It did not mention chlorine, but the Observatory said two people were killed in the strike and others suffered breathing difficulties.

An anonymous official from Syrian President Bashar Assad's government dismissed the allegations of a chlorine attack as fabrications by the rebel forces.

Amid the crisis, 15 doctors in Aleppo sent a letter to President Obama sharply criticizing what they characterized as U.S. inaction.

"Continued U.S. inaction to protect the civilians of Syria means that our plight is being wilfully tolerated by those in the international corridors of power. The burden of responsibility for the crimes of the Syrian government and its Russian ally must therefore be shared by those, including the United States, who allow them to continue," the letter read.

"We do not need tears or sympathy or even prayers, we need your action," the doctors exhorted.

The White House confirmed it had received the letter and released a statement later Thursday.

"The U.S. has repeatedly condemned indiscriminate bombing of medical facilities by the Assad regime in Aleppo and elsewhere in Syria. These attacks are appalling and must cease. We commend the bravery of medical professionals across Syria who are working every day in perilous circumstances with minimal supplies to save lives. The U.S. is working continuously to address the crisis in Syria working through the UN and engaging with Russia and others to find a diplomatic approach to reduce the violence in a sustainable way and allow unimpeded lifesaving humanitarian assistance into areas like Aleppo. As we have seen in and around Aleppo the last several days it is clear the fighting will not stop while both the regime and the opposition fight to encircle one another. A diplomatic solution is required."‎